text:
Why do some helicopters have the tail rotor mounted higher than the tailboom?
Why do some helicopters have the tail rotor mounted high above the tailboom, whereas others have it at the end of the tail boom?
Above the tailboom:

Wiki Commons
At the end of the tailboom:

Boeing.com
I know the reason for having tail rotors, that's not the question at all. I am just curious as to whether the heavier helicopters have different torque counter requirements than the lighter ones?
Generally I notice that the heavier ones have the tail rotor mounted higher up and the lighter ones have it on the boom.
In R/C almost all of the 3D  models have the tail rotor at the same level as the tailboom.

by "centerline" you mean the tail boom?

@Federico I would think in line with the centre of mass which is probably roughly in line with the tail boom.

Correct, i do mean tail boom level roughly

Wow.  There's nothing like a simple question - and this is nothing like a simple question ;) I'll try to find an hour or so to give you an answer tomorrow from a dumb pilot point of view. Also, check out more Comanche images.  You'll see that the tail rotor is canted, from memory, about 20 degrees.

In addition to the excellent answers you already got, I suspect safety may also be a consideration. When a helicopter is on the ground, a rotating tail rotor could easily injure or kill a person. Mounting it higher up, or covering it up, would reduce that risk somewhat. That would be a bigger concern with larger helicopters that typically have more inexperienced people (passengers, cargo handlers,etc) nearby. So it makes sense just for that reason to put the rotor up a bit higher. Just a guess, though.

I've been thinking hard about this question, how to answer it and indeed, whether to answer it or not at all since the question itself is a good one but borders on "too broad".
The answer from aeroalias is good but doesn't offer any "whys". Tail rotor design is also a fiendishly complex subject and, like most things in aviation, is the result of many compromises and inter-dependencies.
As an example of how complex it is, there are two helicopter dynamics "bibles" by Ray Prouty and Walter Wagtendonk. Neither of them fully explain tail rotors and indeed, Prouty often says "for reasons I don't understand". If Prouty doesn't understand something, then you are on a safe bet to say that the majority of helicopter pilots don't understand it either. You need a Physics PhD to design helicopters, you don't need a PhD to fly one.
So this is from the understanding of a pilot without the PhD. I will leave out any mention of which way the main rotor rotates and on which side the tail rotor is mounted since it will needlessly complicate the already complex. I am also assuming a generic "standard" helicopter with a single main rotor and both vertical and horizontal stabilisers. Of course, there are designs that deviate from this but to cover all scenarios would need a book, not an answer on SE.
I'm deliberately ignoring a bunch of other dependencies such as "is the advancing blade of the tail rotor at the front or the rear?"; "is it a tractor or pusher tail rotor?"; the interactions between the main rotor down-wash and vortexes and wind and what happens in high yaw rate situations (fast pedal turns) and sideways flight. Yes, tail rotor design is probably the most complex subject in helicopter dynamics.

As you know, the tail rotor counters the torque applied to the fuselage by the rotation of the main rotor. Since the tail rotor generates lift in the horizontal axis, it also produces a force which will move the helicopter across the ground (tail rotor drift).
To offset this, the rotor disc is rigged such that it tilts in the direction of this movement to introduce a horizontal component of lift which opposes the tail rotor drift and keeps the helicopter steady in the hover. If not rigged to do this with the cyclic neutral, then the pilot will subconsciously hold a little lateral cyclic to hold a steady hover. It's subconscious because you do not hold attitude and position by positioning the cyclic. You position the cyclic to hold attitude and position.
So we now have two horizontal thrust components, one from the main rotor and one from the tail rotor. Now all forces act around the centre of gravity, and it is from there that all accelerations to the aircraft to move it are measured. Therefore, if the centres of these two forces are not inline with respect to the CoG, i.e. the same vertical distance from the CoG, then a rolling "couple" is created between them and the fuselage will roll until the opposing forces are equal and opposite. This is why many helicopters hover with one skid low.
As the aircraft moves into cruise flight, the vertical stabiliser increasingly offsets the main rotor torque so the power required by the tail rotor and the angle of attack needed to offset torque reduces, therefore the roll couple force reduces and the deck of the fuselage will roll back towards level. The horizontal stabiliser also generates lift which further counters the roll such that in cruise, you are using little, if any pedal.
This is our main compromise. If the horizontal components of the lift vectors are not aligned with each other, then you accept an uneven deck in the hover. If the design calls for long periods of hovering (police work, search and rescue etc), then your design is likely to be that both centres are aligned and are above the CoG (the main rotor thrust vector must always be above the CoG because of the engine and main rotor gearbox). It is my guess that the Apache has the tail rotor mounted high to align the centres to provide a level and stable deck which makes sense on a gun platform designed to hover for long periods.
If you look at a model where the tail rotor and main rotor vectors are not centred, for example the R22, then they do indeed hover with one skid significantly lower than the other. This also has the effect of adding a couple of feet to the safe hovering height since catching the skid on the ground whilst rotating or moving sideways is almost certainly a generator of Bad Days(TM).
The trade off is that to position that rotor at the top of the fin requires an additional gearbox and drive shaft at the base of the fin adding weight, complexity and maintenance requirements. Not a concern for the Apache; certainly is for the R22.
Why does the Comanche mount the tail rotor so low, at the end of the boom? Well, I'm not privy to the design decisions and I can't find any authoritative references to them so I am using guesses, which I hope are educated guesses.
One design goal of the Comanche which is not present in almost any other helicopter is stealth.
Rotor blades make great RADAR targets, especially the tail rotor. It's relatively short blades rotating at high speed and stuck way out there in the open provide a lot of strong RADAR returns. There's not much you can do about the main rotor except in the blade profiles and materials (the details of which I expect are classified) but by mounting the tail rotor closer to the main body at the end of the boom and in a fenestron, those RADAR returns will be significantly reduced.
The tail rotor air flows are also going to help to disturb and cool down the engine exhausts by blowing them sideways and mixing them more quickly with the cooler ambient atmosphere. If you look closely, you'll see that the tail rotor is mounted directly aft of the exhausts. I am further guessing that the tail rotor gearboxes are good sources of heat so removing one (by mounting the tail rotor low) also contributes to stealthiness. The added benefit of mounting in a fenestron is to protect the tail rotor from tree strikes and so on, always a concern in nap of the Earth flying.
Because that tail rotor is mounted so low, there is a strong rolling couple and indeed, the Comanche flew very much skid low.

(Source)
To reduce this tendency, the tail rotor is canted (all this from memory) by about 20 degrees which reduces the horizontal thrust vector component and produces around 1000 lbs of lift through the vertical component which offsets the tail low tendency produced by having the TR below the CoG. That high dual horizontal stabiliser is also going to produce lift to keep the tail up in the cruise as the TR power reduces.
From Peter Kämpf:

The 20° canting of the whole vertical also removes a powerful RADAR reflector. Flying low, a perfectly vertical tail would return horizontally oriented radiation, while the 20° angle lets one side reflect RADAR energy into the ground and the other into space.

There is a lot more to all this but as I say, it really needs a book to start to fully address all the factors but hopefully, this answer walks through the main lines of thought.

Thanks for this excellent answer, it is very helpful and makes a lot of sense

@ymb1 Thanks for the edit. As to the side chase, yes, if you are moving in the opposite direction to the rotor thrust (and no, if opposite). There is always a limit to sideways flight though since the tail rotor can enter vortex ring state, move through a null zone with zero thrust and eventually, enter autorotation where the thrust is negative and the airflow is coming up (in the horizontal sense) through the disc. If the angle of attack is maintained with pedal, then the blades can stall and a divergent yaw set in.  Either scenario is bad.

The 20° canting of the whole vertical was to remove a powerful radar reflector. Flying low, a perfectly vertical tail would return horizontally oriented radiation, while the 20° angle lets one side reflect radar energy into the ground and the other into space.

@PeterKämpf Good point, thanks!  3 benefits from one change.  Do you mind if I edit it into the answer?

@Simon Not at all. The goal on SE is to collectively come up with the best possible answer.

There are multiple considerations to be taken in account while determining the location of the tail rotor hub relative to the main rotor plane. One important consideration is how it affects the tail rotor thrust- lesser the thrust required (at higher or lower position), lesser the weight penalty, as the solidity and size requirements reduce.
If we go by this consideration, in general, the tail rotor hub should be located on the plane of rotation of the main rotor. from Tail Rotor Design Guide:

To minimize the collective pitch and solidity required for a given yaw maneuver, to minimize the rudder pedal excursions during a hover turn, and to minimize the power requirements,the tail rotor hub should be located in the plane of rotation of the main rotor and located aft and as close to the main rotor as practical limitations allow.

But this is only part of the story- there are other considerations here:

The interaction between the tail and main rotor has to be accounted for, which may change the location of the tail rotor.

If you are going to have unconventional tail rotors like the fenestron, the tail rotor usually has to sit on the tail boom rather than the fin.

Having them on the same plane usually means mounting the tail rotor high on the vertical fin, which increases weight penalty- you have to have a strengthened vertical fin along with an extra gearbox and driving mechanism; it becomes a tradeoff- at what point does the extra weight worth having the tail rotor on vertical fin. In case of small helicopters, this would mean that it is better to have the tail rotor in the tail boom rather than vertical fin, though there is no hard and fast rule.

The location of the tail rotor can affect the flight attitude- if the tail rotor hub is not along the cg, there is a slight tendency to roll, which has to be corrected by applying lateral cyclic, which means that the heli flies a bit tilted.

There are other considerations- keeping the tail rotor low may increase chances of damage due to debris, while keeping it up in the fin will incur a maintenance penalty.

So, in the end, this is a tradeoff.

"located aft and as close to the main rotor as practical limitations allow" seems odd to me. The farther from the rotor it is, the longer the lever arm and thus, the less energy the tail rotor would need to counteract the torque. At least in my very much not a PhD in physics mind...

Raymond Prouty was the designer for the Apache AH-64 tail rotor (your first picture). In the pre-design phase, the AH-64 had both a T-tail like in your photo of the Comanche, and later on the final configuration in your photo. In this document (behind a pay wall, sorry) he mentions that left sideward flight was smoother and the Dutch roll made more stable when the tail rotor was raised during the flight test development phase - purely aerodynamic considerations.
In the second picture, the Comanche shows a T-tail and a tail rotor in a fenestron: tail rotor and horizontal stabiliser have swapped location. Locating the tail rotor in a fenestron has several advantages: the vertical fin does not block the thrust, and it is shielded from the external flow and rotor wake in forward flight. A fan-in-fin is only possible at tail boom height, so if this design is chosen the height follows automatically.
Raising the tail rotor complicates the mechanical linkage and therefore is associated with higher power losses, but this is not listed as a dominant pre-design feature in the two reference books that I have. Aerodynamic considerations seem to carry more weight here: interference with the main rotor in hover, sideways flight and forward flight. With military helicopters, operational requirements for tail strike have influence on placement of the tail rotor as well.
Placement of tail rotor also affects lateral trim in the hover, depending on what kind of main rotor is applied. From Helicopter Performance, Stability and Control by Raymond Prouty:

As you can see, only the teetering rotor with the tail rotor at height of main hovers horizontally. But hovering with one skid down seems to be not a big problem.

In reviewing the other answers I'm seeing a lot of detail, conjecture about weight and other important related effects but not really a lot of direct answers.
background
I play a little known game called Kerbal Space Program and within it I am I think the only person to design a functional conventional single main rotor helicopter with a realistic articulated rotor system. The demonstrator Helicopter I am currently updating to publish along side my updated comprehensive guide (available on the KSP forums) is a high tail rotor design like the Apache.
Why I'm here
The reason I ended up here is to find corroborating evidence from a more official source, for the viability of lower tail rotor designs; as I am considering tackling them next. So below I'll give my speculation as to the design principles in having either a high or lower tail rotor, trying to directly answer the question.
As I see it there are three possible tail rotor configuration: high, low and middle (neautral). Whilst the tail rotor has other effects, such as horizontal thrust and counter torque, I'm not going to discuss them much.
general
I believe tail rotor height is to bias the roll of helicopter across a variety of flight regimes, depending upon what is import speed or stability.
specific
A high tail rotor (pusher prop) slight above Centre of Gravity induces a roll to the right, a low tail rotor below CoG would induce a roll to the left and a rotor in the neutral point shouldn't induce roll in either direction.
A high tail would need to produce more thrust and a low less thrust to induce roll, obviously this has some design implications for speed, torque and drag that I'm not going into here but it might be an engineering factor which determines the configuration you go with.
explanation
Why would you want intentionally induced roll in either direction? Why not have all tail rotors be roll neutral? In short, it's a way of mitigating dissymmetry of lift across range of flight speeds.
The effective trust of the tail rotor is variable. If the tail rotor (high or low) is roll-biased, then at higher speeds you can vary the trust of the tail rotor, to induce a roll in order to counter dissymmetry of lift.
(Dissymmetry of lift being the differential of lift between the retreating and advancing blades of the main rotor which causes an asymmetric roll on the retreating blade side as the helicopter increases in speed)
You might be wondering
How is stability achieved at low speeds or in a hover with a tail rotor that is heavily biased above or below the CoG?
Well it's done in a similar way to the neutralisation of horizontal thrust produced by the tail rotor, in which the main rotor is canted to produce a slight horizontal thrust in the opposite direction.
To counter tail rotor induced roll, the main rotor is offset slightly to once side. The main rotor is effective lifting the helicopter off-centre of the CoG, this is true in both high/low configurations but in opposing directions.
short answer
Both high and low tail rotors are a way of dealing with dissymmetry of lift at a variety of speeds and flight regimes but I figure a low tail rotor is direct affect of high speed design over low speed stability.
long answer
There are numerous engineering considerations for a high/low design. The blade tips on helicopters reach supersonic way before the helicopter ever could, so it's possible that having a tail rotor that produces less thrust and possibly spins at a lower rpm is a better choice at high speeds. Also at very high speed, counter-torque is partially achieved via the air-frame itself, although more counter-torque is required due to increased main rotor drag and torque transmission to the stator, this can be leveraged to require less work from tail rotor.
Some advanced designs actually reduce main rotor RPM relying on fixed wings to provide some lift, allowing the main rotor to remain effective even when approaching the supersonic limit for rotor blades; so that they might continue to work in conjunction with a reduced need for counter-torque.
Finally, high and low tail rotors counter dissymmetry of lift through opposing methods. In a high configuration the tail rotor bias is directly countering dissymmetry by producing more thrust and rolling in the opposite direction. In a low config the tail is effectively aiding dissymmetry, and it's the main rotor which therefore counters dissymmetry; the tail rotor effectively just being a counter balance to the intentional asymmetry of the main rotor.

I don't think physics from KSP should be included in an answer here.

As it’s currently written, your answer is unclear. Please [edit] to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers in the help center.

I'll review my answer to be more clear however I'm going to do more research first. I'm confident that my answer better addresses the question than the previous answers I've read. As for "I don't think physics from KSP should be included" I didn't reference KSP physics, that is simply my motivation for researching real helicopter physics, so I can apply what I learn. As to speculation, I have neither a Apache nor Comanche helicopter, nor their manuals to hand. Like anyone else I can only work from reasoned assumptions, and apply what I know about the physics of flight to them. repost (errors)

I'll add what sources I find to my answer but here just one: https://www.spinningwing.com/the-helicopter/helicopter-forces-and-moments
"The vertical location of the tail rotor is typically above the helicopter’s CG, so that it’s lateral thrust pushes the upper portion of the helicopter left, rolling it left side down" "The main rotor counter’s the roll moment due to the tail rotor to keep the helicopter upright" Removed references to diagrams not present in this comment but the diagrams can be found at the linked address. Consider this part one of the "what" I'll working on the "why later.


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What is the difference between "Land 2" and "Land 3" annunciations for Boeing autopilot systems?
Another question's answers refer to certain Boeing jets having the capability to annunciate "Land 2" and "Land 3" when flying a coupled ILS approach. What does each of these modes require, and allow, beyond the basic coupled ILS? How are they different from one another?

From "Smart Cockpit"
B777 Automatic Flight (PDF!)

AFDS Status Annunciation
  The following AFDS status annunciations are displayed just above the PFD attitude display:
   • FLT DIR (the flight director is ON and the autopilots are not engaged)
   •   A/P (the autopilots are engaged)
  •   LAND 3 (three autopilots are engaged and operating normally for an
  automatic landing)
  •   LAND 2 (AFDS redundancy is reduced; in some cases, only two autopilots are available)
  •   NO AUTOLAND (the AFDS is unable to make an automatic landing).
  With a LAND 3 indication, the autopilot system level of redundancy is such that a single fault cannot prevent the autopilot system from making an automatic landing (fail operational).
  With a LAND 2 indication, the level of redundancy is such that a single fault cannot cause a significant deviation from the flight path (fail passive).
  An EICAS message is displayed for any fault which limits the capability of the automatic landing system. Aural alerts for EICAS messages not affecting safety of flight are inhibited until after touchdown. Changes in autoland status below 200 feet, other than a transition to
  NO AUTOLAND status, are inhibited.
From Chapter 9, pages 22-23

This is great information, but if at all possible please post text rather than images: searching won't find text information in an image. If that isn't possible here then adding a brief summary/explanation to the image would be a good alternative.

So, Land3 with all AP working, gives you Cat III autoland, right? And Land2 is good to, what, Cat II mins but you still have to make the landing? Is there a reason beside an A/P failure what you'd be in, or prefer to be in, Land2 instead of Land3? Are these modes needed for anything besides Cat II and Cat III mins?  (Other than exercising the system, currency, etc.)

Land 2 is still good for a Cat III Autotland but with a Decision Height of 50'.      There are numerous other equipment failures that can cause Land 2 status. (Autothrottle, Pitot heat, ILS receiver,Hydraulics etc).

There is very little correlation with the Land 3 and CAT III.

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sort: votes


text:
What first aid equipment is on a commercial aircraft?
I have always wondered what type of first aid equipment is on a commercial aircraft with an airline such as Thomas Cook or Ryanair. Do they have things to save your life etc.??

Obviously, first aid kits have items that save your life. The contents of the First aid kits or Emergency medical kits (EMKs) vary based on the jurisdiction. For example, FAA requires the following items

... on all airplanes of air carriers operating under part 121 for which at least one flight attendant is required. EMKs and AEDs are “no-go” items and must be carried as indicated on the Minimum Equipment list.

Contents of EMK; table from FAA Advisory Circular 121-33B Emergency Medical Equipment.
On the other hand, Indian DGCA requires the following items in first aid kits:

First Aid contents; table from DGCA Document No: F No. 11-690/CAR X-II/2010/AI(2) Provision of Medical Supplies in Aircraft
Most of the requirements are broadly similar, while there are some region (or country) specific differences; for example, DGCA requires rehydration solution to be carried, presumably due to heat. FAA, on the other hand requires Automated External Defibrillators (AED) to be carried for aircraft with payload more than 7500 lbs (DGCA requires it only in international flights of over 2 hours).

I've often wondered if FAA required AED's

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text:
Why is there a provision of pressure setting in GPS?
Some of the GPS have a provision of setting barometric pressure (QNH/QFE). How does it work?

Welcome to the site, can you post an actual model so we know what you're talking about?

Related


text:
What is Waterline and why is it called so?
What is waterline and why is it called so?  Is it same as fuselage reference line?

Water lines (WL) designate location in the height of the aircraft, from ground up.
This is explained more in this question


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Why does the canopy of the F-22 Raptor have a golden tinge?
Why is gold used in the canopy of the F-22 Raptor?

This question was originally answered here: http://aviation.stackexchange.com/a/25435/7394

In order to reduce the radar cross section (RCS) of the aircraft, the F22 canopies are coated with a thin layer of Indium Tin Oxide (ITO), which gives it a golden tint.
From the book F-22 Raptor by Bill Sweetman:

The F22 Raptor canopy ... comprises two sheets of polycarbonate, sandwiched between two layers of optical glass, fusion bonded in an autoclave and drape formed over a canopy blank ... A metallic coating of indium-tin-oxide is added to the canopy to reflect the radar waves, giving it a golden tint.

They are used in stealth aircraft to reduce the RCS. From globalsecurity.org:

A window member composed of a transparent resin or inorganic glass with a transparent conducting film such as gold or ITO (indium tin oxide) coated thereon, is used as an electromagnetic wave shield window for stealth aircraft. Applying such transparent conducting film enables, while maintaining transparency to visible radiation, both a radio wave stealth property which scatters radio waves in various directions so as not to be detected by radar, and an electromagnetic wave shield property which prevents harmful electromagnetic waves, except for visible radiation, from invasion into an aircraft.

This technique is used in other aircraft like the EA6B Prowler and the F-35.

It seems strange that radar waves are reflected in order to reduce the radar signature.

In the EA-6B the purpose was to shield the occupants from the RF energy the jamming pods were putting out.  And it was actually gold film.

@mins, the key is that the incident waves are partly absorbed and partly re-emitted; this process produces a radar return that is "scattered" in a range of directions most of which do not point back to the radar receiver. Think of light reflecting off a mirror versus a bumpy white  flat surface: the mirror produces a beam in one direction while the white surface produces a diffuse glow.

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Why is horizontal stabilizer set to 4° up in airbus aircraft for take off?
Why is horizontal stabilizer set to 4° up ( or any other setting other than neutral) in airbus aircraft for take off? Why can't we keep the trim neutral?

On the EMB-145 we use 8 deg up, occasionally 7

What is the source of your info?

Airbus WILL take off ~safely~ regardless of the pitch trim, as long as trim is in the t/o green range.
Adjusting the trim to the correct position, however, depending on the location of the CG for that flight, insures the aircraft handles similarly between takeoffs and rotation will be more or less standard between flights. This means the pilot can make the same side-stick input on each takeoff and get the same 3deg/sec rotation.
If not properly trimmed, rotation would be faster/slower and the pilot would have to adjust for that at a critical phase of flight. Not doing a good rotation will impact t/o performance significantly.

Both landing and take-off are critical phases of flight, and it is standard procedure to configure the aircraft for those phases to reduce the workload on the crew.
On landing, aircraft are configured for a stabilized approach, which means that the aircraft can be flown to the runway with a minimum of configuration changes to flaps/slats, trim, power, and drag devices.
On take-off, the aircraft is configured for take-off, and trim is set so as to provide a slight nose-up. If the trim is set nose low, it requires a lot of pull to "unstick" the aircraft from the runway. If the trim is set too nose high, then the aircraft will want to nose up before the aircraft may be ready to fly, and then to continue a nose-up attitude after rotation, requiring more than a minimal effort to keep the a proper departure attitude.
Nose-up trim can induce what is called a trim stall, which is when the trim is set so far back that the aircraft will exceed the critical angle on take-off:

ELEVATOR TRIM STALL
  The elevator trim stall maneuver shows what can happen
  when full power is applied for a go-around and
  positive control of the airplane is not maintained.
  [Figure 4-8] Such a situation may occur during a
  go-around procedure from a normal landing approach
  or a simulated forced landing approach, or
  immediately after a takeoff. The objective of the
  demonstration is to show the importance of making
  smooth power applications, overcoming strong trim
  forces and maintaining positive control of the airplane
  to hold safe flight attitudes, and using proper and
  timely trim techniques.
Source: Airplane Flying Handbook p. 4-10

The idea is to trim the airplane for the initial climb phase, so it assumes the correct attitude and speed without further control input. The exact value depends on CG location, weight, flap setting, temperature and more.
Consider you take off in IMC with nose-down trim and get the least bit distracted. The aircraft is at a real risk to hit the ground.

I don't see what the KLM Flight 633 accident has to do with trim settings.

@JonathanWalters It has. With more nose-up trim, the aircraft would had climbed even with the gear extended. Add to that pilot confusion and no visibility (it was dark), and you have exactly the situation I described.

I see no evidence to support your conclusions. The accident report cites pilot scan error during flap retraction coupled with an inadvertent gear re-extension. A poorly timed power reduction seems to have also been a contributing factor. The aircraft struck the water tail low. There is no indication of nose down trim. Aircraft are often trimmed for Vy, which may not account for sink during flap retraction or for extended gear.


text:
Why is a strake placed on the fuselage in front of the tailplane on the Hawk aircraft?
Why is a strake placed on the fuselage in front of the tailplane on the Hawk aircraft?

Maybe this image would be better, as we can see the surface. Also you should provide a link to the page containing the image.

They are called Tailplane Canard Vanes (TCV) or the side mounted rear fins (smurfs). Basically, they are vortex generators to solve the problem of tailplane under surface stall at high (negative) angles.
During flight testing of the Hawk, it was found that the tailplane under surface stall was causing uncontrollable nose down pitch. From The Hawk Story:

It was found that at forward centre of gravity in that configuration, rapid fore-and-aft movement of the control column could induce an uncontrollable nose down pitch, with the nose down attitude and speed increasing quite rapidly. ... It was dubbed the “Phantom Dive”.

The designers fixed it initially by removing the the outboard vane of the flap; however, a better solution was needed and this was achieved by placing a tailplane canard vane in front of the stabilizer. The TCVs created a voretx, that ensured that the control surfaces remained effective by preventing seperation. The image below shows the improvement.

Image from The Hawk Story:
The addition of TCVs solved the problem of tail plane under surface stall. From the same document:

With tail and TCV on, there is ample tailplane authority with aft stick (- $\eta_{T}$ ) at all angles of attack in the usable range.
The addition of TCVs was a cheap and effective solution of the problem of tailplane under surface stall, and allowed further development of the wing to try to achieve the low stalling speeds demanded for carrier operation in the US Navy, and for combat versions of the aircraft carrying heavy store loads.

The paper has quite a lot of additional details on solving this problem, and I suggest you go through it. According to RAF Aircraft & Weapons Guide, the modificaiton was incorporated on the Hawk T2.

You may have noted that the tailplane of the Hawk is a full-flying surface, also called an all-moving surface. When the stick is pulled aft, the leading edge will align with the strake, forming in effect a leading edge extension. This will produce a vortex over the lower surface of the inner tailplane and prevent early flow separation at the tailplane root at low speed.
The obvious next question is: Why is it not part of the tailplane itself? Adding it to the tailplane would increase drag and produce nonlinear control forces. Since it was added later in the life of the design, fixing it to the fuselage allowed to keep the tail surfaces and their actuation unchanged.
Note that early Hawk versions (Mk.1, Hawk 50) lacked this detail. In flight testing, it was found that the Hawk would suddenly nose-dive with full flaps and gear up. This was dubbed "phantom dive". Cutting back the flaps solved that behavior, but later, heavier versions of the Hawk needed more lift, so a different fix had to be found. From the Hawk story by Harry Fraser-Mitchell:

It was shown with the half model of the Hawk at Hatfield that high
  local downwash at the tail, coupled with the very large nose down
  pitching moment induced by the flap, was causing the tailplane to
  stall on its lower surface, so that it could no longer provide
  adequate balancing power. It needed more lift, extended to higher
  angles of attack.
In the case of the F-4K Phantom aircraft this was achieved by
  installing a fixed leading edge slot to the tailplane, harking back to
  demonstrations of such devices by Handley Page on wings in the early
  Twenties! A fixed slot with its associated drag was not an option on
  the Hawk although a cambered tailplane was tried on the model with
  some success. Removal of the outboard vane of the flap reduced the
  flap pitching moment to such a value that the standard tailplane could
  cope, so this was the quick solution for the RAF. However, for the US
  Navy VTX project, (and for later combat versions of the Hawk) the
  maximum possible lift was required, so that at least the outer flap
  vane had to be replaced. The dive phenomenon had to be fixed.
An example of the cross-fertilization of knowledge due to the matrix
  working of the design department now occurred. Barry Pegram, then
  Section leader of the Fluid Dynamics section of the Aerodynamics
  Department, had been working on the adoption of leading edge root
  extensions (LERX) for the “Harrier” wing, and this work had shown that
  these devices extended the lift of the wing to higher angles of attack
  by virtue of the non-linear lift developed by the vortex flow they
  created. He proposed that these should be added to the tailplane of
  the Hawk model, but this would have had a serious effect on the
  tailplane hinge moments. The author, who was working with Barry in the
  V/STOL tunnel at Hatfield, suggested that the ‘tailplane canard vane’
  (TCV), as it was called, could be fixed to the fuselage at such a
  position that it was lined up with the flow at normal conditions, but
  with its trailing edge adjacent to, with a small clearance, the
  leading edge of the tailplane at its maximum nose down position.
Experimenting showed that these vanes could be made quite small and
  they gave a complete cure to the problem in the wind tunnel, with very
  little drag in the normal flight regime. To prove the concept in
  flight, some temporary vanes were manufactured which could rapidly be
  fitted to one of the test aircraft which had flaps with the full vane.
  First, the aircraft was flown without the TCV to establish the
  conditions under which the ‘Phantom Dive’ occurred on that particular
  aircraft. On a later flight, the TCVs were fitted and the aircraft
  flown again to the identical conditions as before. Despite every
  attempt by the pilot to instigate the phenomenon, it did not occur –
  the vanes were a complete success, even though they looked
  inconspicuously small for such a large effect.


text:
Is the term "guinea pig" defined in the Pilot/Controller Glossary?
Is the term "guinea pig" now or was it previously defined in the Pilot/Controller Glossary?
If it's not defined in the Pilot/Controller Glossary, maybe it's in some other publication.
I was flying with a crew member who used it while querying ATC about threading a hole in a line of weather. It went something like, "has anyone else guinea pigged that hole?" He said it was an actual controller term in their documentation.
I can't find it, but I was wondering if there was some kernel of truth to the statement.

Being a "guinea pig" means to be the first to try out a (potentially dangerous) situation. There is no mention, past or current of the term "guinea pig" or any derivation of that.
Slang slips through on the radio, this is probably an instance of that. If the pilot or controller used that term the appropriate response would be "unclear, please say again".
There is also no mention of it in the AIM.

Does the standard RT phraseology cater for that specific situation, though? Haven’t flown in ages so am a tad rusty...

@CptReynolds I don't think so, but it could be in much clearer English.

I heard a new one yesterday. Flight following for someone was cancelled as an airport was neared, the pilot was told to squawk VFR and frequency change was approved. The response back was "loading the dozens", changing to local frequency etc.  I hadn't heard that for changing transponder code to 1200 before.

@CrossRoads I would think that isn't just improper, but a bit dangerous. As an observer on the frequency I might think he was loading "1212" (dozens).

As an IFR rated pilot, I would agree. ATC didn't question him on it or correct him. The direction to squawk VFR puts the reply in the right context tho.

"Loading the dozens" doesn't have any advantage over "squawk VFR."  On top of being non-standard, ambiguous, and too cute by half, it even takes longer to say.

People are in aviation for different reasons. But I think the biggest motivator of slang in the professional pilot world is boredom. For a VFR pilot it’s probably motivated by insecurity stemming from never getting more than 110 knots across the ground. I’m being humorous here, but really.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
How do the flight decks work in unison on the Stratolaunch?
This article implies that the Stratolaunch has two flight decks. How are the dual flight decks arranged on the Stratolaunch aircraft?

I'm curious if things are fly by wire or if there are separate controls?
Are the flight decks both fully operational and mirror images of each other?
If there are duplicate switches and controls, how do the counterparts reflect changes initiated from the cross side?

I'm pretty sure the flight deck is only in one of the two fuselages I am trying to find some info to confirm before posting an answer.

On Stratolaunch Systems, there will only be one cockpit as mentioned here:

The pilot, co-pilot and flight engineer will be in the right fuselage
  cockpit, while the left fuselage one is empty and unpressurized.

News can be misleading. This article's title says that each fuselage has its own cockpit but later, it clarifies:

The left-hand fuselage has what looks like a cockpit with windows to
  onlookers, but the section is in fact empty and unpressurised.

It does look like there both fuselages have cockpits.

Image Source

That was the exact article that spawned the question. And I obviously was too lazy to finish reading it. ;) Thanks.

In stratolaunch, the only the right fuselage houses the flight crew i.e. the cockpit. From stratolaunch.com:

The right fuselage houses the flight crew and the left fuselage contains flight data systems.

Stratolaunch, image from bubblemania.fr; original appears to be from stratolaunch.com


text:
Why is the surge line curved?
Why is the surge line of an aeroengine curved? Why is it not a straight line?

Please provide the source for your image

Surge is a very complex phenomena, coming for flow going in opposite direction to the main flow. That's is a very complex phenomena, the question is why do you think a such a complex phenomena is having a linera behaviour

Simply put, it's supposed to follow a constant flow/fanspeed curve but multiple stages and the mechanics of surging make it a lot more complex in practice.

The surge line slope on multistage compressors can range from a simple, single-parabolical [sic] relationship to a complex curve containing several break-points or even 'notches.'  The complexity of the surge line shape depends on whether or not the flow limiting stage changes with operating speed from one compression stage to another; in particular, very closely matched stage combinations frequently exhibit complex surge lines... The surge line in many cases has an approximately constant value of Q/N [mass flow/fan speed].  In many multi-stage units, and especially in units which are controlled by variable inlet guide vanes, the surge line curves bend dramatically at higher flows and speeds.Source: "Practical Aspects of Centrifugal Compressor Surge and Surge Control" by  Meherwan P. Boyce (pdf)

So some of the shape comes from the curvature and irregular spacing of the constant speed lines, and some of the curve comes from adding in different stages.
Unfortunately, exact surge conditions are not easy to describe mathematically even if we know the properties of each stage in the compressor.  There are things like multi-blade interactions and boundary layer effect that make things difficult really fast.  These factors are so important because surge is not quite the same as a stall on compressor blades.  Surge is a more complex unstable compressor flow resulting from stalls. Single blade stall, which isn't observed much in practice, will not cause a surge.  Surge happens when the stall propagates across several blades due to blocked incoming air.  There aren't easy,exact, and universal equations for this complex behavior unfortunately.

I "quotified" the quote to make it easier to identify. Feel free to revert or adjust if I missed some of it.


text:
What are the differences between ACARS and CPDLC?
What are the differences between Aircraft Communication, Addressing and Reporting System (ACARS), and Controller Pilot Data Link Communication (CPDLC)?
Can anyone please let me know in short, what is the difference between ACARS and CPDLC? So far, I have learned that both use data link communication. A ground station is necessary for ACARS and I am not sure weather ground station in necessary for CPDLC or not. ACARS can works on VHF and HF frequency also on SATCOM. Does CPDLC also works on these or it requires different system to work on? CPDLC gives the messages on a digital display. Does ACARS also provide information on a digital display or it is like fax machine where information come out on a paper from the machine.
I become confuse while figure out the differences between ACARS and CPDLC. Can anyone help me out?

Nice question! I think that CPDLC is the generic term: one implementation of the CPDLC is through the ACARS system (that is a trademark name). But I wait for a complete answer, since I am interested like you as well.

@GianniAlessandro  That is incorrect. CPDLC is much newer than ACARS. Neither is an implementation of the other.

While @ymb1 has provided a comprehensive answer, I’ll offer a simpler one:
CPDLC is an application, which can run over multiple networks (including ACARS).
ACARS is a network, which supports multiple applications (including CPDLC) and runs over multiple data links.
Applications tend to be screen- and keyboard-oriented, but terminals have the ability to print data or pass it between applications. For instance, if the pilot receives a new route in CPDLC (maybe via ACARS), they can press a button to send it to the FMS, rather than have to print it out and then retype it.


text:
Does a subsequent crossing restriction supersede earlier crossing restrictions?
Does a subsequent crossing restriction supersede earlier crossing restrictions?
Here's an example.
While flying the MIP4 arrival into KLGA, you receive the expected instruction to cross MARRC at FL180. Before reaching MARRC, you receive the instruction to cross BILEY at 13,000. These fixes being expected is not material to the example. The question is, when given the subsequent instruction to cross BILEY at 13,000, does this supersede the instruction to cross MARRC at FL180.
If possible, please cite answers in 14 CFR, AIM, or another FAA source.

AIM 4-4-10 (g) gives us guidance on this situation.

The guiding principle is that the last ATC
  clearance has precedence over the previous ATC
  clearance. When the route or altitude in a previously
  issued clearance is amended, the controller will
  restate applicable altitude restrictions. If altitude to
  maintain is changed or restated, whether prior to
  departure or while airborne, and previously issued
  altitude restrictions are omitted, those altitude
  restrictions are canceled, including departure procedures
  and STAR altitude restrictions.

For the example in the question, unless the controller restated the requirement to cross MARCC at FL180, that clearance is cancelled by the issuance of the clearance to cross BILEY at 13,000.


text:
What is the difference between a propeller and a rotor?
I have been making the mistake of using words "propeller" and "rotor" interchangeably when they are not the same thing. What is the difference between a propeller and a rotor?

'at times' ... 'somewhat interchangeable'. When? Can you provide examples? It might help someone like me who's so accustomed to aviation terminology that these terms don't really ever feel interchangeable at all. The only exception is maybe the V-22, which uses what they call proprotors.

I've always understood a rotor to be a horizontal propeller mounted on a helicopter, while a propeller was a vertical rotor mounted on a airplane. i.e., they're the same thing, differing only in the angle at which they're mounted to the craft in question. (I do realize that there tends to be a fair bit of twist in some propeller blades that doesn't seem to occur in rotor blades, but otherwise, same-same.)

@FreeMan A helicopter rotor and a propeller both have aerofoils, fixed to a hub and rotated by an engine.  That's about the end of the similarities. Propellers and rotor heads are far from same-same.  Most aspects of their design are different.

@egid Modified to address your comment, thanks.

@egid: The "rotor" part of the word "Proprotor" is due to the V22's Propellers being able to rotate perpendicular to their main axis, I would think.

Short answer:

A propeller is used to propel something, the force at stake being called thrust. Thrust is usually horizontal (but not always, think about rockets). This device comes from the navy domain.
A rotor of a rotorcraft is used to control anything, including the altitude, the generated force being called lift. Lift is generally vertical (but not always, think about a rudder). Rotors have generally more than two blades.

Lift and thrust involved are components of the same physical force (the aerodynamic force) to which we refer to differently based on their purpose after an arbitrary breakdown based on the direction they act (compared to the direction of the aircraft or to the gravity field).
For our pleasure, let's also talk about a turbine rotor, which is a rotor/propeller which doesn't produce lift or thrust, but convert the fluid velocity (actually momentum) into torque. For example the wind turbine rotor. A turbine rotor is certainly a propeller, but we don't refer to it as a propeller, as it propels nothing.

Rotor is the most generic name for something spun by a shaft. It can be used for a propeller, the rotating wing of an helicopter, the rotating elements of a compressor or a turbine, etc. You can't be wrong using rotor for any spinning device. Rotor (rotating) is the antonym of stator (stationary). These terms are not specific to aerodynamics, in a electric motor there is usually a stator and a rotor as well.
The rotor of a rotorcraft comprises blades and is comparable to a propeller, if we except blades additional freedom in flapping and leading-lagging to limit vibrations created by repeated force swings encountered while performing a 360° turn.

Source: Wikipedia
But actually the main rotor is a (rotating) wing, and the anti-torque rotor is a (rotating) rudder. Their blades are meant to produce lift like the wing and the rudder they replace. Because lift is involved, we tend to name them rotors rather than propellers.
A propeller also comprises blades, fixed on the rotation axis.

Source.
The purpose of a propeller is to propel the aircraft, that is to push it in the direction of its flight.
Both a rotorcraft rotor, a propeller and a wing create the same aerodynamic force. However the conventions are:

A rotor and a wing are used to eventually counter weight and control altitude, and a propeller to eventually counter drag and control velocity along the flight path (I say eventually because to gain altitude you usually temporary increase thrust, that's a detail)
A rotor and a wing create lift, a propeller creates thrust.

Wing, propeller blade and rotorcraft blades work exactly the same from an aerodynamic standpoint. The resultant of their action is the total aerodynamic force, which is then broken down arbitrarily into lift, drag, thrust and torque to facilitate mechanical analysis.
Notes:

Moving an aircraft along its path (thrust) is easier than maintaining its altitude (lift), and therefore a main rotor is far larger than a propeller for aircraft of the same mass.
The horizontal displacement of the rotorcraft is also produced by the main rotor (cyclic control), so some part of the aerodynamic force can also be seen as thrust, and the rotor also acts as a propeller.
Propeller is used for simple propellers and not in turbine engines. While the fan of a turbofan is comparable to a propeller, participates to thrust production and aircraft propulsion, the fan is still only one part of a system (guide vanes, fan duct and exhaust nozzle greatly maximize final thrust). Bottom line the fan is a rotor, not a propeller.

Hmm. (Some) Rotor blades (rotorcraft) are not fixed, some are.  Depending on the head design, they may or may not have flapping and lead-lag hinges.

@Simon: You're the master of rotorcraft :-) I mean they can lead-lag thanks to their flexibility. Correct?

Exactly.  In a fixed head, the forces from flapping and drag (lead/lag) are absorbed by the blades flexing. Plus 1 anyway ;)

And what would be the most correct term or terms for the non- flapping fixed-blade rotors or propellers used on radio-controlled quadcopter models, and on manned aircraft designed along similar principles?

@quietflyer: As long as these elements are wings, they are called rotary wings, rotating wings or rotor wings. The rotor being the axis and the blades. As said, a part of the force generated is used for propulsion too, but we usually don't refer to them as propellers. Some do, especially when the rotor has only two blades. This is of no consequence as anyway there is only one principle involved: Creating an aerodynamic force using rotating blades.

@mins What produces thrust in the rotorcraft?  Isnt the rotor itself provides thrust too?

@Auberron: You're right, that's explained in the answer, and also in the previous comment.

Rotor is a generic term for a rotating part of a mechanical device.
Fan is a rotor that creates a flow within a fluid (i.e. gas or liquid).
Propeller is a fan with the purpose to create thrust.

According to these definitions, it is not wrong to name propellers as rotors and vice versa.
In aviation language usually helicopters have rotors and planes have propellers, because:

A plane propeller creates (mostly) thrust, and the lift is created (mostly) by the wings.
The air flow from a helicopter rotor generates both, lift and thrust.
Helicopters are differentiated from planes by rotary wings versus fixed wings.

In general a rotor is anything that requires continuous rotation to function. Rotors also occur in devices such as the distributor in an electrical ignition system and in some rotary pumps.
The vertical-axis rotary wing, used for lift by helicopters and gyroplanes, is commonly called a rotor. Horizontal-axis lifting rotors also exist; the flanged cylindrical Flettner rotor and the flat wing rotor are two examples of Magnus rotors. More complex horizontal-axis lifting rotors include the cyclogyro and FanWing, both examples of the cross-flow fan.
A spinning set of blades designed to create gas flow is often called a fan. Such fans are typically housed in a duct.
A spinning set of blades designed to create thrust for forward motion is often called a propeller. But this is not so far from the function of a fan, so here we get into niceties, such as;

Where a ship's propeller typically has very broad short blades, an aircraft propeller typically has long thin ones.
Where a propeller typically has a small number of blades, a fan typically has a large number. For aircraft this is sometimes expressed as disc coverage; if you look through the rotor disc and see mostly the far scenery then it is a propeller but if you see mostly blades then it is a fan (note that it does not apply to marine propellers).
The ducted front stage of a high-bypass turbofan engine, which provides much of the thrust, is called a fan. This contrasts with the free-air propeller of the turboprop.
A piston engine with a ducted propeller is likely to have it called a ducted propulsor.
Some modern turbine engines have multi-blade thrusters curved to avoid compressibility effects and spinning in free air; these have sometimes in the past been described a ductless fans, although Europrop call them propellers.

I trust that is wholly clear and unambiguous to you! Or, to put it another way, the industry just makes it up as it goes along and uses whatever word seems like a good idea at the time. You just have to remember what they have decided for each gadget.

Propeller is getting lift from the wing and thrust from the propeller but helicopter gets both from the rotor.
propeller aircraft can not hover over a spot but helicopter can.
Propeller aircraft can not perform vertical landing but helicopter can do.
Propeller blade is pre-twisted but helicopter blade are symmetrical.
Propeller aircraft has two hard/re-enforcing point such as: Wing root and Engine mount but helicopter has only one hard point that is "ROTOR MAST" and more differences are there.

Please don't use a signature, tagline or similar. If you want to display your credentials, you can do so in your profile. See What kind of behavior is expected of users?, specifically the point on Do not use signature, taglines, or greetings.

This answers a different question: "list some differences between airplanes and helicopters."


text:
Does having lead-in lights in sight allow descent below MDA?
Does having lead-in lights in-site during an approach that includes them allow for a pilot to descend to 100 feet above the touchdown zone elevation? 14 CFR 91.175 allows a pilot to make such a descent if the approach light system is in-site, but does a lead-in light system qualify?
The approach also states the following:

When visual reference established, fly visual to airport via lead-in lights to Rwy 13L or 13R.

Also worth referencing here is this question about what counts as runway in sight, but this question is specific to lead-in lights and is not answered directly by that question.

Possible duplicate of What counts as "runway in sight" when flying an IFR approach to minimums?

I doubt this is a duplicate, the question essentially asks "Is the lead-in light system considered [a part of] the approach light system?"

This is not a duplicate. The question marked as the answer does not answer the question in reference to lead-in lights. This is a special situation. But I guess there's value in having the reference at the top.

No, you may not descend below MDA having only the lead-in lights in sight.
Reference FAA Order 6850.2b, "Visual Guidance Lighting Systems", page 1-3 through 1-5.
According to this order, the list of approach lighting systems is as follows:

Medium Intensity Approach Lighting System (MALS).

Medium Intensity Approach Lighting System with Sequenced Flashers (MALSF).

Medium Intensity Approach Lighting System with Runway Alignment Indicator Lights (MALSR).

Simplified Short Approach Lighting System with Runway Alignment Indicator
Lights (SSALR).

High Intensity ALS with Sequenced Flashers (ALSF-2).

Omnidirectional Approach Lighting System (ODALS).

The Lead-In Light System is not in this list.  Instead, it is found further down the page.  Here's the key sentence:

The LDIN may be terminated at any approved approach lighting system, or it may be terminated at a distance from the threshold compatible with authorized visibility minimums permitting visual reference to the runway environment.

The LDIN is not an approach lighting system and so you may not use it to descend below MDA.

Given that the MAP is 3.6 NM to 13L or 2.6 NM to 13R and the minimums are 800/2nm for Cat A or B it stands to reason that you can continue from the MAP to the runway with only the LDIN in sight. You may not, however, descend below MDA until the runway is in sight. That is how I understand it.

According to the FAR-AIM, RLLS is one of the 9 types of approach light systems. It seems that this would allow you to descend to 100’ under 91.175 guidance.

I suspect that the other answer by Steve V. may be correct, but this post raises the valid point that the FAR-AIM lists RLLS alongside the other systems like ALSF-2 and MALSR and so on. If this analysis is wrong, let's find the explanation why, since S.E. policy is to downvote, rather than to delete, incorrect answers. This post was never about raising a new question (though making a point with a rhetorical question can come off as that).


text:
Can large airliners do aerobatics such as loops?
This subject has been partially covered before here Is it possible to do a barrel roll in a large aircraft like a B737? and here Is it safe to roll an airplane that isn't approved for it?
If a airliner such as say a 777-300 had enough height, could it theoretically do a loop or similar aerobatics?
I am supposing that there is no passengers, luggage, hostesses and minimum fuel on board etc - so minimum weight.
Also, taking into account the loop - if it couldn't achieve the loop by pulling up - could it achieve it by going nose down instead?
IE: The aircraft goes nose down till it is upside down then pulls up - a mirror version of a normal loop basically.
The other questions go over the reasons why not to roll a large aircraft, I want to know if other aerobatics (specifically loops) are theoretically possible.

You'd have to disable the envelope protection first though.

Not a duplicate I think - the question is asking specifically about either inside or outside loops, which have different limitations.

@PeterKämpf the others only go over rolls - I want to know about other aerobatics - specifically loops

NOT a duplicate question, because the answers are different!  Roll, yes, it's a 1-g maneuver. Loop, likely no because of the higher g-loading.  How in the world is that a duplicate question???

I don't know why everybody only talk about the B737 barrel roll, but Concorde seems to have performed it too.

Read up on Federal Express Flight 705. When a crew member attempted to murder the pilots and crash the plane (a DC-10), the pilots (who were former Navy/Air Force) basically used the aircraft as a weapon to throw the hijacker about the plane and keep him incapacitated. It's an amazing story! There's a Seconds From Disaster episode on it too.

@JMK: Are you sure about that? I think they did not film an episode of Seconds From Disaster about it, but it was featured on Mayday: Air Crash Investigation, Season 3, Episode 4: http://en.wikipedia.org/wiki/Federal_Express_Flight_705 It's a good one.

Maybe it was Mayday, one of those shows anyway

Not exactly what you're asking, but: https://www.avgeekery.com/time-boeing-717-went-inverted-testing/

I found this on the newjs. https://www.youtube.com/watch?v=LeYVtVg6aGs

Design
The key point is that these vehicles are not designed to perform such maneuvers.
A coast-guard lifeboat is designed to self-recover from a capsize and can roll through 360 degrees without significant structural damage. A cruise-liner would not survive being rolled over completely.
If a vehicle is not designed to perform a high stress maneuver it is safe to assume an unmodified vehicle cannot do so without incurring severe damage.
Inside loop
Regarding an inside loop, the world's smartest man says

Looping a 747 or a DC-10 would be trickier [than rolling one ...]  Boeing suspects its planes could make it, but since no one has ever been silly enough to try, there's no way of knowing for sure.

Outside loop
An outside loop is much much more stressful on airframe and pilots as it involves negative G. Airliners have flight envelopes that have much less capability under negative G. There must be a very high probability the aircraft would break up when attempting an outside loop.
Roll
A barrel roll is a less stressful maneuver.
60 years ago, in 1955, "Tex" Johnston, a Boeing test pilot performed a barrel roll in a Boeing 707. So far as I can find out, no one has since deliberately attempted to replicate this stunt in a commercial airliner. At the time of publishing his autobiography, the pilot claimed in a newspaper report that the stunt was planned by him but unknown to his boss.
in 1985 China Airlines 006, a 747, inadvertently executed a half roll but suffered structural damage. It rolled to 60 degrees before the pilot disengaged the autopilot, the pilots suffered spatial disorientation and the roll continued into a dive. The pilots obviously weren't flying the aircraft within its limits - meaning you can't read a lot into this, other than it might be easy to break up the aircraft while attempting this sort of maneuver.

Note that the same article states: "there once was a Boeing test pilot who, in a moment of frivolity, took it into his head to execute a barrel roll in a 707. He made it" and then regarding the 747 (not 707) "The consensus at Boeing seems to be that a 747 would probably survive a barrel roll"

@JoeBlow: Barrel roll is special in that it is low positive G throughout and the only unusual condition is moderate amount of side-slip. So it does not put too much stress on the airframe.

@JoeBlow: IIRC the barrel roll in 707 was some demo, so the "moment of frivolity" was more like "we can't obviously approve it, but it would be cool, so we are going to pretend we don't know what you are planning".

@Jan: Good point, I should have included that incident. - I have now.

A German registered 707 crashed in 1964 on a training flight during an attempt to fly a (barrel?) roll. Apparently one roll was successfully completed, but crew lost control of the second one: https://aviation-safety.net/database/record.php?id=19640715-0

@CptReynolds To be pedantic: that accident was a Boeing 720, not a 707.

@MichaelK Thank you and my apologies for the lapse! Can’t edit the comment, I‘m afraid.

@CptReynolds Just delete and re-post... pretend like it was never there. :D

@MichaelK Will do next time the PC is on. Can’t paste into comments in the iOS app, apparently...

Probably not, due to structural strength issues. A barrel roll is one thing; they can be performed at a positive load factor through the whole maneuver (assuming sufficient control authority).
Loops on the other hand will need much greater loadings in the "fast" portions in order to keep the loop "round" and to allow the maneuver to be completed without stalling.
Inverted loops are even more extreme as the aircraft is accelerating into the maneuver, requiring potentially greater loads on the airframe.
Outside loops... might as well forget it, the negative G-Loads are more or less guaranteed to cause an airliner airframe to fail; negative Limit Load Factor for Part 25 cert requirements is -1 G and Ultimate Load Factor is -1.5.
Who knows, maybe Boeing over-engineered enough to allow a 74- to take the 4+ Gs it would need to in order to complete a loop, but I'm skeptical they could build that much strength into the structure, for weight reasons if nothing else.

It's going to be hard to provide a hard answer, because it would vary from aircraft to aircraft.  But, let's take a Boeing 737 as a reference, simply because it's quite common, and small(ish), so one would assume it would be more likely to survive than a larger jetliner.
According to this guide on executing a loop (pdf), you want to maintain roughly 3.5 positive g's when executing a loop (and this is best case, in the hands of an experience pilot.)  Bearing that in mind, Boeing lists the maximum positive g's for a Boeing 737 at +2.5g (with flaps retracted).  Which leaves us an entire unit of earth's gravity short of being able to safely execute the maneuver.
So, in the case of the 737, the answer is a resolute no.  And I think it's safe to assume that most commercial aircraft are not much stronger (In fact, in doing some more checking the Boeing 747-400 and the Airbus A320 have the exact same g load characteristics, here and here respectively.)
So we can probably assume that no commercial jetliner can safely handle a loop.

The 2.5G seems to be certification requirement. And designing the plane stronger makes no sense, it would make it heavier.

+2.5G is the positive Limit Load Factor, i.e. high risk of permanent deformation of the airframe if exceeded. The Ultimate Load Factor is 150% of the LLF and also part of certification requirements; the danger in exceeding it is that there is a substantial risk of outright airframe failure.

There was an incident where a B757 unintentionally flew a 3.5G manoeuvre without  serious damage - in fact it was allowed to remain in commercial service for several months afterwards without a technical inspection, until Boeing did some further analysis of the flight data recorder. See "Icelandair Flight 315" at https://en.wikipedia.org/wiki/Icelandair#Accidents_and_incidents

@alephzero Nifty, nice find.  Though, as always, I feel obligated to point out that "one incident doesn't make a sample size".  They were probably lucky more than anything else...

Airplanes can sustain more positive G forces than negative G forces because of their construction. Your described opposite loop is only for very advanced aerobatic airplanes and pilots because negative G forces are involved. I think normal loops are possible if performed by a skilled pilot like a test pilot, but are too risky or too expensive (in case of failure) to try.  A Boeing 777 costs $300 million.  Who will want to over-stress an airframe that costs millions just for a loop?!

They are not designed for aerobatics. Design limit loads of 2.5g are too low for performing inside loops. Ultimate load is 1.5 times limit load = 3.75 g, which could be enough for an inside loop if expertly done, eating into the safety margin and playing with structural failure and death.
But that is with a standard plane, at full load. An aeroplane specifically kitted out might be able to do an inside loop:

The best candidate would be a shortened version of an aircraft, like the B747SP or the F70: wing loading is less than design load which results in additional g tolerance.
Structural loads on the wings could be reduced further by installing tip weights.
The aircraft must be stripped of all unnecessary items such as passenger seats to make it as light as possible.

The manoeuvre loads should be kept inside the limit load, leaving the 1.5 factor to the ultimate load as the safety margin it is intended for. It should be possible to achieve an ultimate load of around 6g (limit load of 4g) for a modified plane, given what happened to China Airlines flight 006. Flight control authority needs to be verified, and all structural elements including the tail, fuselage, flight control surface hinges etc. that were designed to the same loading specs will need to be scrutinised and possibly modified to make sure they don't fail at the higher load.
But with such a modified plane and an expert pilot, it should be possible, yeah.

Check out Tex Johnston's 707 prototype barrel roll.  While it isn't large by today's standards, it did involve a plane that became an airliner.

Tex's barrel roll was the first thing that came to mind when seeing this question.  This is a classic moment of aviation history.

Tex Johnston's (note spelling) barrel roll is implicitly acknowledged in the question by linking to other questions that discuss it. The question specifically asks about "a loop or similar aerobatics" and, as the other answers point out, a barrel roll isn't at all similar to a loop.


text:
Why did the B-29 have so much leftward torque at take-off?
In this book, The Making of the Atomic Bomb by Richard Rhodes (p. 705), I read:

[Paul Tibbets] eased the brakes [of Enola Gay] at 0245, the four fuel-injected Wright Cyclone engines pounding. “The B-29 has lots of torque in take-off,” he notes. “It wants to swerve off the runway to the left. The average mass-production pilot offsets torque by braking his right wheels. It's a rough ride, you lose ten miles an hour and you delay the take-off.” Nothing so rude for Tibbets. “Pilots of the 509th Group were taught to cancel torque by leading in with the left engines, advancing throttles ahead of the right engines. At eighty miles an hour, you get full rudder control, advance the right-hand engine to full power and, in a moment, you're airborne.”

I am not especially knowledgeable in aviation matters, but I believe that the torque is due to the B-29's four propellers all rotating in the same direction. Is that right? If so:

Why did this problem only arise during take-off?
Why did all propellers rotate the same way? Is this true for all propeller-driven aircraft with more than one engine? Why?
If the 509th Group's technique was so good (and apparently not especially complex), why did only they use it?

Although this particular technique was not, in fact, unique to the 509th (as per @JonStory's answer), one response for as to why, hypothetically, the 509th might adopt some technique not used by other B-29 crews could be that the 509th operated Silverplate/Saddletree (nuclear-capable) B-29s, which were rather extensively modified for nuclear delivery, whereas most B-29 crews flew non-nuclear-capable aircraft.

It didn't arise ONLY during takeoff, but it is most noticeable then for several reasons

The engines are at maximum power, therefore producing maximum torque
You've got a very good reason to need to be travelling in an exactly straight line (the runway)
You're travelling at fairly slow speed, so your vertical stabilizer is less effective - at higher speeds, the tail counteracts the torque more effectively (for this B-29 this occurs at 80mph, as mentioned). As soon as you get above 80mph, you can just input a little right rudder to counteract it.... B-29's only travelled below 80mph on takeoff and landing, so it wasn't a problem during flight, and during landing the throttles were at idle or very low power.

It isn't true for all aircraft with more than one engine now (although it is true for some), but it does still happen and was much common back in WWII - they didn't want the extra supply line complexity of two different sets of otherwise identical engines, when it could be solved with a bootfull of right foot. Note also that the B-29 had 4 engines, most modern multi-props only have two, and they're closer to the fuselage, which reduces the effect
I think you're misinterpreting the source: he's not saying this technique was unique to the 509th, just that they were taught the technique. The very fact he says they were taught it, rather than "came up with" it, suggests it was known elsewhere. The 509th are only referenced here because that's the group Tibbets was part of, it's unrelated.

P-factor is actually something different, that comes into effect on rotation and climb out due to the prop blades hitting the air stream at an angle. On the takeoff run you're only looking at torque and perhaps prop slipstream.

Thanks for the explanations. Regarding point 3., the source also says that “the average mass-production pilot” did it in the different, less efficient way.

falstro - good point, corrected. @DaG, the average mass production pilot isn't the same as a bomber pilot. I suspect the technique just hadn't, in wartime, transitioned over to non-military pilots. As mentioned, the B-29 was particularly "bad" for torque, as it was very powerful. The DC-3, for example, wasn't as badly affected: they just usually aimed slightly to the right on the runway and used rudder/aileron control or a tap of the brakes to keep it manageable until the rudder properly became effective

@DaG That means one of the instructors of the 509th knew of using diff-thrust for yaw control at low speeds and passed it on to his students

Thanks, Jon, I see. I thought Tibbets were implying he trained his group in a special way.

It's likely that different units would have independently arrived at the same technique, it's just common sense.

@JonStory "The average mass-production pilot" I read as the average B-29 pilot, who was always a bomber pilot. The thing I read is "average people are taught that in flight training, but the good pilots know to use diff thrust. The 509th only had good pilots, so we used diff thrust."

Very minor point: Placing the engines close to the fuselage has no effect on the torque due to engine rotation. It does, however, make a big difference to the engine out behaviour.

Copper hat, that's a bit of a relic from when I (mistakenly) mentioned p-factor regarding the aircraft still on the runway, which has now been removed

Additional note on the second question: An example for a modern multi-rotor aircraft using counter-rotating propellers is the Airbus A400M - it uses a different gearbox setup in two of the engines to achieve this. In contrast, the C-130's Allison T56 and C-130J's not quite as ancient RR AE 2100 engines turn all propellers in the same direction. The downside of the reverser gearing is, of course, additional weight in two of the engines (27 kg in the TP400 case).
Source: http://www.easa.europa.eu/system/files/dfu/EASA-TCDS-E.033_Europrop_International_GmbH_TP400--D6_engine-04-04092013.pdf

I recently had the change to speak with a current co-pilot of the CAF B-29 "Fifi".  He stated that the pilot or co-pilot would typically advance the throttles for takeoff with his hand on the levers in a "twisted" position so that any given lever would be further forward than all its neighbors to the right, i.e. the tops of the levers would form a diagonal line from forward left to aft right.  So differential thrust is used to compensate for a left-turning tendency.  He stated that you wouldn't use full throttle at this point, because then you'd have nothing remaining to work with-- this implies that a correction to the left might sometimes be needed, since a further correction to the right could always be accomplished by reducing the right-hand throttles.  He stated that once the rudder became effective, the pilot flying the airplane would then call for the flight engineer to "set takeoff power", at which point all the throttles would be advanced to the same position.
This pilot was somewhat skeptical of the idea that it would be common technique for a pilot in any group flying the B-29 to use the brakes to keep the airplane going straight during the take-off roll.  He said it would be considered bad technique today in the CAF, due to the risk of skidding a tire and leaving a flat spot.
The pilot's general description of the technique he used, and the specific point that  some "room" was left to allow corrections to the left as well as to the right, suggests that the left-turning tendency in the B-29 on takeoff wasn't too extreme.  However, since the plane didn't have a steerable nosewheel, some other form of compensation was required.

Moving on from the specific comments by this pilot, to some general observations about left-turning tendencies on take-off in tricycle-gear aircraft that sit in a level attitude until the aircraft is rotated for lift-off--

In general, engine torque creates a rolling (banking) tendency to the left, but the aircraft can't roll (bank) very much when the wheels are on the ground.  However the rolling tendency does increase the weight on the left tire, and thus the rolling friction of the left wheel.

In a tricycle-gear aircraft that sits in a level attitude until it is rotated for lift-off, P-factor  does not create a turning tendency during the portion of the takeoff run that precedes rotation.  In general, P-factor creates a yawing tendency, but P-factor only exists when the aircraft is at a positive angle-of-attack, i.e. when the aircraft is in a nose-high pitch attitude in relation to the direction of the flight path.  This is not the case when the aircraft is in a level attitude, with all the wheels on the ground.

Gyroscopic precession does not create any turning tendency during the portion of the takeoff run that precedes rotation.  In general, gyroscopic precession creates a yawing tendency whenever the aircraft is pitching, i.e. whenever the pitch rotation rate is not zero.  When the propellers rotate clockwise, gyroscopic precession will create a left yaw torque when the nose is pitching up, and will create a right yaw torque when the nose is pitching down.  So gyroscopic precession will make an aircraft with clockwise-rotating propellers yaw to the left as it is being rotated for takeoff, but not before or after that point.

During the portion of the takeoff run that precedes rotation, the spiralling (helical) slipstream (propwash) does exert a force on the vertical fin.  When the propellers rotate clockwise, the upper part of this spiral is moving toward the right, which exerts a rightwards force on the vertical fin, yawing the nose toward the left.  This force will be greater at lower airspeed than at high airspeed, since when the airspeed is low, the propwash or slipstream will tend to dominate over the free relative wind created by the aircraft's motion through the airmass.  Even though the vertical fin of the B-29 was not directly centered behind any one of the engines, the airflow over the vertical fin surely had some degree of left-to-right component which undoubtedly contributed to the aircraft's left-turning tendency on take-off.

Harvey S Plourde's excellent book "The Compleat Taildragger Pilot" (1991) contains a pair of graphs showing the individual and combined effects of all the different factors that contribute a left-yawing tendency during the takeoff roll in a propeller-powered aircraft with clockwise-turning propellers, for both the tailwheel case and the tricycle gear case, from the start of the takeoff roll all the way to the moment of liftoff. Different factors are dominating at different points in the takeoff roll.

Now to address one other aspect of the original question:
In the vast majority of twin-engine or multi-engine propeller-powered aircraft, all the propellers turn the same way.  As another answer has noted, this simplifies production, and the supply of replacement parts.  Of the well-known American-made twin- or multi-engine aircraft built in large numbers during the second world war, only the P-38 Lightning had propellers rotating in opposing directions.

Note-- the links embedded in this answer go to sections in John S Denker's excellent "See How It Flies" website.
Note-- all references to "clockwise" in this answer are from the perspective of someone standing behind the aircraft looking forward, as is the accepted convention in aviation, presumably because it also relates to what the pilot sees when looking forward through the windscreen at the propeller of a single-engine airplane of the "tractor" configuration.
Related ASE answers--
What is the influence of yaw on P-factor?

Torque does contribute a bit of turning tendency by putting more load on the left wheel than the right one (given a clockwise propeller rotation). More load means more rolling resistance

If the props are to blame, it might be worth noting that, in addition to more powerful engines, the silverplates had different props.

Re "He stated that you wouldn't use full throttle at this point, because then you'd have nothing remaining to work with-- this implies that a correction to the left might sometimes be needed, since a further correction to the right could always be accomplished by reducing the right-hand throttles. " -- I'm no longer convinced that the logic here is accurate.  With the throttles in the "diagonal" configuration described, and the whole cluster "full forward" so that the left-most throttle was firewalled, you could still steer in either direction by retarding the leftmost or rightmost throttle.

(Ctd)  On the other hand, if the whole diagonal "cluster" is to be moved as a single unit, it seems logical that there will tend to be a right-hand steering effect if the "cluster" is well aft so that the left-most throttle is near idle, and likewise if the torque is "balanced" with the "cluster" at some intermediate position, then it seems logical that if the whole "cluster" is moved in unison further forward to increase power, there would tend to be a net left-wards torque.

(Ctd) In other words the diagonal line of the "cluster" would need to be made more diagonal if the pilot wanted to move the throttles forward without creating any torque effect, but the pilot is not doing that, he's just sliding the whole "cluster" forward and aft as a unit, so you have a rightward torque effect when the cluster is further aft and a leftwards torque effect when the cluster is further forward.

(Ctd) Answer has already seen several edits so need to give some thought as to how convey this new "twist" on things most clearly before trying to make another change ...

Something about the Tibbets statement does not ring true.
I have talked to dozens of B-29 pilots and not one of them mentioned having to do either of those things during takeoff: stepping on the brakes or advancing the left engines first. Stepping on the brakes seems like an especially horrible idea. The crews were flying overloaded planes that needed every inch of runway just to get in the air. The last thing they would want to do is slow the aircraft down by 10 mph during the takeoff run.
Also, prior to being assigned to the 509th, Tibbets was involved in training B-29 crews in Nebraska. Since most, if not all training flights would have been conducted in lighter B-29s, the tendency should have been even greater. Did Tibbets learn the technique there? Did he teach it there? (The pilots I talked to were trained in Nebraska while Tibbets was there.)
It could be that this was a problem that was unique to the Silverplate aircraft operated by the 509th because they had more powerful engines than regular B-29s.
I will check with some people who flew/fly B-29s including one pilot who flew both regular and Silverplate aircraft in 1945.

Right, at low speed (with little wing lift) the wheels also should have some say in where the airplane is going. Braking is not the only option.


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Why do the engines seem to power down and then back up after take-off?
What is the phenomenon on a commercial airliner (say, a 737-800) where shortly after takeoff and before we reach 10,000 feet the engines feel like they slow down dramatically or even power off momentarily, then spin back up?
This happens fairly frequently when I'm flying and, although I'm used to it, it still takes me by surprise every time!

Do you mean the engines immediately spool back up or after a while?

There are several kinds of power reduction which could happen after takeoff:

Cutback (noise reduction when the airplane is very low, after takeoff)
Climb thrust

Both are set on the CDU (Control Display Unit, the screen and keyboard of the FMC, Flight Management Computer).
Since you describe a temporary and dramatical power decrease, this should be the cutback. When you hear the power increase again, it must be that the setting goes from cutback to climb thrust.
In the B737-800
The cutback option on the CDU

This is the CDU (Control Display Unit), where you set up the cutback. It is in the second page of TAKEOFF REF. Here is the screen before turning cutback ON.

ACCEL HT : This is the height (above ground level) at which we will let the plane accelerate (we will lower the pitch a little bit and gradually retract flaps).
EO ACCEL HT: Same as above, but applies when one engine is not running (Engine Out Accelaration Height). This setting will be used in case of engine failure after takeoff.
REDUCTION: This is the height at which the thrust setting will go from TOGA to CLIMB. We need maximal thrust during takeoff, because it's a critical phase and it's safer. But at the same time, keeping this setting too long decreases the engines lifetime because of the high temperature. It also burns more fuel, it's noisier and less environment-friendly. That's the reason why the airplane won't climb with maximum thrust.

But this question is about cutback. Here is CDU again after turning cutback ON.

CUTBACK N1: This will be the target N1 setting when the REDUCTION height will be reached. The Auto-throttle together with the EEC will arrange for maintaining this lower shaft speed (here you can see it's 84,6%). This will be a lot less noisy but will still provide enough power to climb!
EO ACCEL HT: Same as above
REDUCTION: At this height, cutback will happen and CUTBACK N1 will be the engines target N1 speed.
RESTORE: This is the height at which cutback is no longer needed, and climb thrust N1 setting will be the new N1 target for the engines (92.3% in this example).

Takeoff with cutback

This is the takeoff roll. IAS indicator is not active yet (below 45KIAS). Ground Speed is currently 35kt.
N1 and TO/GA are displayed on the FMA (Flight Mode Annunciator) at the top of the PFD (Primary Flight Display). On the right, you can see on the ED (Engine Display) that the green marker is indicating TO thrust (because of TO/GA mode).

Few seconds after the rotation. You can see at the bottom of the PFD the radio altimeter indicating 300ft AGL. We haven't reached the REDUCTION height, so the cutback is not active yet. The ED still indicates TO thrust and the green markers are just below 100% N1.

Few seconds after reaching REDUCTION height. N1 is active on the FMA. Radio altimeter indicates 960ft AGL, this is above our REDUCTION height, so the cutback should be active and our engines should be a little quieter. A look at the ED and we can see that the green target N1 marker is now indicating 82.6% N1. Our engines are still rotating faster than that (98% N1), and the EEC is currently decreasing the fuel flow arriving to the combustion chambers.

We are now reaching 2500ft AGL. This is below the RESTORE height of 3000ft, so the cutback is still active, as shown by the ED. The green target N1 marker is indicating 83.0% N1 and the current N1 for both engines is also 83.0% N1. In 500ft, the N1 target speed should increase again to climb thrust settings.

We've just reached the RESTORE height (above 3000ft AGL), and we can see the green markers show 92.3%. We are back to the climb thrust settings and our plane is loud again! The flaps are still down, and the pink speed marker on the PFD just moved to the top. We are going to retract the flaps and slats, and accelerate to 250KIAS. It seems that RESTORE height is also the ACCELERATION height when cutback mode has been turned on. [I'm not sure, could a B737 pilot confirm this?]

The flaps are up, our speed is now 250KIAS.

Further readings

http://www.boeing.com/commercial/aeromagazine/aero_21/quietclimb_story.html
http://www.flaps2approach.com/journal/2013/6/29/avoiding-confusion-using-the-following-acceleration-height-t.html

In addition to noise reduction aspect I suspect there are human factors involved. On takeoff and the initial climb you are exposed to a continual acceleration which you become accustomed to. The reduction to climb thrust could feel like a dramatic reduction as the forward acceleration diminishes.

That makes a lot of sense.  The plane needs high engine thrust to get off the ground but once airborne and low, it also doesn't want to disturb nearby residences, so power would be cut back to enough to maintain a climb.  Once high enough where engine noise isn't a problem, climbing thrust would kick in.  Sweet!

:) I will edit my answer in several hours to add a screenshot of cutback option details in the B737 NG.

Thank you - that makes perfect sense and it's what I thought was the case!

Why would the engines cutback at FL 100?

@Fox can't wait for that screenshot ;)

@shortstheory We don't use pressure-altitude to trigger cutback, but height above ground level (radio altimeter) instead. What counts is your distance to the surface of the Earth, because the farer you are from the ground, the less noisy you are. Usually, cutback happens much much lower than 10000ft AGL (around 800ft AGL).

@CompuChip I just added the screenshots and some explanations about the cutback feature in the B737-800!

Wow this is fantastic. Thanks for going the extra mile!!

Takeoff clearances are received from Tower ATC and contain an altitude limit.  "Delta 632, cleared for takeoff runway 32L climb and maintain 3,000."  Upon reaching 3.000 the pilot must level off which means he must throttle back.  Then he will receive a further climb clearance from Departure Control and in most cases that clearance will be to a higher altitude.  The pilot must throttle up for that.
Noise abatement procedures also affect this as well.
When at a non-busy airport ATC may clear the plane higher before he levels off, in which case the climb will be continuous.
The throttle down you notice is routine and normal.

This would be under FAA jurisdiction, as take-off clearances in ICAO-land do not contain altitude restriction, but rather have an initial climb altitude on the SID chart. And airplanes are usually cleared higher before reaching the initial climb altitude, as it's in many cases some 4000-5000ft above airport elevation. Additionally, the OP does not really specify that a level off occured.

"When at a non-busy airport ATC may clear the plane higher before he levels off" - and at busy airports as well (from my experience at EHAM). In my opinion initial clearances are mostly formalities related to a handover: both so controllers stay away from each other's airspace and to provide a safe situation in case the next controller doesn't issue the next clearance in time. But in practice ATC's secondary objective is to make flights as smooth as possible and providing clearance well in advance so pilots don't have to take unnecessary actions is part of standard procedure.

I'm sure someone will have a better answer, but I think its when the aircraft moves from a take off power setting to a climb power setting.

There's no hurry to have an answer posted immediately so why not just wait for somebody to post the better answer?

@DavidRicherby I think "to gain rep" would be a very valid answer, especially when someone is new at stackexchange.

I have no interest in "gaining rep" actually, I answered because I thought it might help answer the question. But with the attitude people have I think it would put anyone off trying to pass any knowledge they might have.

Because the engines in some of these aircraft are more powerful than needed. Remember, 250 airspeed rule below 10k feet. If an aircraft is cleared to "after takeoff climb and maintain 2000" then he wont need all of this power. Additionally, the aircraft isn't fighting drag from gear/flaps at this point, nor friction on the ground. And it is no longer a race to get off the runway. Why not save fuel AND avoid a stern talking to from the FAA at the same time? Once the aircraft is cleared to a higher altitude, they spool up again for the climb.

"Because the engines in some of these aircraft are more powerful than needed."  Engineers don't design planes to be overpowered.  Yes, certain runways/load conditions/weather situations make the plane overpowered FOR THAT SITUATION.  FAA regulations require that transport category aircraft must be able to maintain a certain climb gradient at max gross weight with one engine inoperative (OEI).  If you do the math, this means a twin engine airliner will lose about 80+% of its climb ability with OEI.  Single engine performance determines how powerful the engines have to be...  (cont.)

So my argument here is that no certified airliner is overpowered, but I agree that in certain situations a plane may have excess thrust for a given situation.  Sorry for picking this giant nit...


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What was the Space Shuttle's glide ratio?
The Space Shuttles glided to a landing in the atmosphere after reentry. They were not good gliders, aerodynamically speaking, so their glide slope angle was rather steep.
What was the glide ratio of the Space Shuttles in this phase of flight?

You should clarify what you mean by "in this phase of flight". The shuttle was constantly slowing down throughout reentry and landing, and its L/D changed as its speed changed.

After the so-called "alignment cone", the glide slope is minimal, about 20-22°. This can be guessed from the landing systems on the SLF runway: MBLS, and PAPI plus Ball-Bar, at 20°. Here the glideslope in the HUD (source Youtube, STS-129 landing).

@mins it's always 20° on final (or 18° for orbiters returning with heavy payloads). I would agree that "cone" should be in quotes since it's not technically a cone, though it was called that.

The NASA website quotes an approximate ratio of 1

The shuttle was designed with a low L/D ratio (~ 1) because during the descent the spacecraft must be slowed from about 17,300 mph to about 250 mph at landing.
  (Source)

Note that glide ratio is normally equal to the lift to drag ratio - Wikipedia article - Aviation.SE Question

This; they wanted drag, because drag was what allowed the spacecraft to slow down. Orbital velocity is blazing fast; you don't want to hit a runway that way, or even the thicker parts of the atmosphere (which in this case amounts to a few tens of km altitude). Compare how the Space Shuttle flew S-patterns during reentry to bleed off speed.

The L/D increased substantially as it slowed down, especially in the subsonic region. You would need at least a 45° glide slope on approach with a L/D ratio of 1:1. The shuttle flew at either 18° or 20° on approach (depending on weight), and it did so on the front side of the L/D curve.

@MichaelKjörling sorry, it's just a nitpick I have anytime I hear the shuttle flew s-turns "to bleed off speed". That makes it sound like the point of the s-turns were to bleed off speed, and they weren't. The shuttle needed to use bank angle to control drag (by way of controlling descent rate). The s-turns simply existed to compensate for the fact that this method of drag control caused them to turn off course. I wrote a whole answer on it here.

As far as I know the space shuttle is no longer in use.  However, the glide ratios (more than one in different configurations) can be found on the Wikipedia page: Space Shuttle Wikipedia page.  There it states that “The orbiter's maximum glide ratio/lift-to-drag ratio varies considerably with speed, ranging from 1:1 at hypersonic speeds, 2:1 at supersonic speeds and reaching 4.5:1 at subsonic speeds during approach and landing.”

"No longer in use" is an understatement.  The last shuttle launch was in 2011, and the orbiters themselves were decommissioned and rendered unfit for flight.

This paper apparently answers your question.

Eyeballiny the figure (I haven't read the paper itself):
Mach 4 ≈ 2
Mach 3 ≈ 2.5
Mach 2 ≈ 3
Mach 1 ≈ 4
Mach 0.6 ≈ 5

So far the most correct answer. Funnily enough, the selected answer is the least correct...